Scott asks what is a book, why is harder / easier to write than a chapbook (etym, "cheap book"), etc.
There are by and large three sorts of poetry I've come across in a chapbook: collections, series, and long poems. Kim Lyons' "In Padua" and Rachel Loden's "The Last Campaign" are collections (Loden's is I'd argue a series as well). Loden's won the SH prize (beautifully produced chapbooks in this series -- hi Stephanie Strickland!) and directly preceded HI. Loden has published several chapbooks. And I even have some of her poems from the 70s in a journal somewhere. So, these collections largely precede the book publication of the same poems, and are generally by poets who are slow to find their way into printing a full length book for a variety of reasons, motherhood included.
The short but complete series is also something good outside of a full length book format -- if it is 30 pages long, why pretend it is not. My forthcoming chap, "Cocktails," on furniture press is an example of that. I wanted all of them to be all together, because they speak to each other, and I figure they are fun, and why wait another two years or more until they make it into OOD.
And, the single piece which is a "long poem" but not a "book length poem" is good. This begins to blur a bit -- I have a series of long poems that have similar sources and work together, but some of the individual poems I've sent out and / or had published as chapbooks. My Belladonna chapbook is an example of that. Some of Rachel Blau Du Plessis' individual Drafts are chapbooks.
So why is writing 30 pages easier than writing 48 pages? Well, because it is, in my experience. Now, it is easy for some poets to write, and it is hard for other poets to write, and this says nothing about the quality of the poetry. Some poets spend more time writing than others. Some poems are easier to write than others. But, in my experience, it is easier to write 30 pages than 48 pages. It takes less time, etc. It is easier to write 60 pages (Locket) than 208 pages (DaDaDa). A chapbook is one poem in OOD, or one series in OOD. Easier to write the part than to write the whole. In DaDaDa, which was four books collapsed into three books, those books were actually separately-written books, and then I did a great deal of rewriting on top of them to get the parts to make sense together.